One proposed power output apparatus sets an input limit and an output limit of a secondary battery according to the observed conditions of the secondary battery (see, for example, Japanese Patent Laid-Open Gazette No. 11-187577). The prior art apparatus sets the input limit and the output limit of the secondary battery according to the observed temperature and the current state of charge of the secondary battery, and drives a motor and a generator within a range of the input limit and the output limit. This prohibits the secondary battery from being charged and discharged to an excessively high level of electric power and prevents deterioration of the secondary battery.